What is a typical usecase of building multi module maven project into one jar?

Question

Some posts in stack overflow explained when we should use multi module project instead of multi single project.

I'm new to the build and deployment.

Now if there is a maven project of several modules( say 5), what is the benefit of building all in one jar instead of just letting it build each jar for every module? Is the main reason that deploy a single jar is much easier than deploy 5 jars or is there some other very important concerns?

Thanks in advance.